1. Field of the Invention
This invention relates generally to orthopaedic devices and more particularly to patello-femoral joint implants and instrumentation.
2. Related Art
The knee joint is a frequent place for joint damage, and the loss of normal (i.e., relatively pain-free) ambulatory function is a frequent result of such damage. Many different causes, or combination of causes, result in knee joint damage. For example, a modest overextension of a knee weakened by osteoporosis can result in damage. Moreover, the extent of the damage to the knee joint can vary greatly depending on the cause, age of the patient, pre-existing conditions and other factors.
The knee is a common source of problems because the joint has an unusually large range of motion and bears nearly half of the weight of the entire body. A primary knee movement, known as flexion-extension movement, includes bending (flexion) and straightening (extension) of the leg in which a lower part of the leg (tibia and fibula bones) flexes in relation to an upper part of the leg (femur bone). Ideally, the knee joint is capable of almost 180 degrees of flexion-extension movement. The knee joint can also accommodate a certain amount of rotational motion in which the lower leg rotates a few degrees in relation to the upper leg. This wide range of motion requires extensive contact surface between the femur and the tibia. Further, the knee joint is rather loosely held together by tendons and ligaments to permit such a wide range of motion.
The front, or anterior side, of the knee joint is protected by the knee cap or patella. The patella is held in place by ligaments and slides over a femoral joint surface during flexion-extension movement. The patella and its ligaments are mechanically involved in joint extension. If any of the joint surfaces (e.g., femoral surface, patellar surface, or tibial surface) becomes damaged or roughened, the knee joint will not operate properly and the patient is likely to experience significant pain.
A common problem is damage to the patello-femoral joint that causes free motion of the patella to be inhibited and/or painful. Such damage is sometimes referred to as “runner's knee.” Patello-femoral joint (PFJ) damage can make normal joint movement almost impossible.
A variety of prosthetic replacements have been developed for different joint surfaces of the knee joint. In extreme cases, the entire joint can be replaced with a prosthetic device. Such a prosthetic replacement is referred to as a total knee replacement. However, total knee replacement requires a considerable time for recovery, and it may be advantageous to replace only the damaged part of the joint in less extreme cases.
In some cases, PFJ damage may be adequately addressed with a PFJ arthroplasty, as opposed to a total knee replacement system. This type of knee surgery is less drastic than total knee replacement. It is designed for patients whose main problems involve only the patello-femoral part of the knee and is directed to providing a smooth sliding relationship between the femur and the patella. The surface of the femur on which the patella slides is referred to as the trochlear groove. The trochlear groove is the indentation or groove located between the medial and lateral condylar surfaces at the distal end of the femur.
In prior art PFJ prosthetic systems, a prosthetic patellar bearing surface is introduced. The prosthetic bearing surface typically includes an anchoring portion for receiving natural patellar remnants. As a result, the final patellar structure includes a posterior prosthetic bearing surface and an anterior natural patella surface. The anterior natural patella surface typically retains the connective tissue that connects the patella to the quadriceps and tibia.
In order to achieve adequate translational movement of the prosthetic patellar bearing surface, particularly in the presence of damage to the trochlear groove, a cooperating prosthetic femur implant is typically affixed onto the end of the femur. The prosthetic femur implant in most cases includes a bearing surface that is specially adapted to receive the prosthetic patellar bearing surface to ensure reliable travel during flexion movement.
Such prior art systems, however, are typically highly artificial systems that employ unnatural patello-femoral tracking or movement of the patella. One drawback of such systems is that they are not compatible with total knee replacement systems. In many cases, the PFJ system requires so significant an amount of bone removal as to render subsequent total knee replacement almost impossible.
More natural patellar devices employ a saddle-shaped design. The saddle-shaped design may be used with or without a femoral implant and is intended to track the within the natural trochlear groove.
There is a need, therefore, for a patello-femoral prosthesis having the advantages of more naturally tracking designs. There is a further need for a femoral implant that requires less bone removal for implantation.